U.S. patent number 10,557,817 [Application Number 15/996,560] was granted by the patent office on 2020-02-11 for method of inspecting electrode provided in gas sensor element.
This patent grant is currently assigned to NGK Insulators, Ltd.. The grantee listed for this patent is NGK INSULATORS, LTD.. Invention is credited to Yuki Nakayama, Taku Okamoto, Soichiro Yoshida.
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United States Patent |
10,557,817 |
Okamoto , et al. |
February 11, 2020 |
Method of inspecting electrode provided in gas sensor element
Abstract
A method of inspecting an electrode provided in a gas sensor
element includes the steps of: producing, in advance, a calibration
curve representing a relation between an Au maldistribution degree
defined based on a ratio of an area of a portion at which Au is
exposed on a noble metal particle surface and calculated from a
result of XPS or AES analysis on an inspection target electrode,
and a predetermined alternative maldistribution degree index
correlated with the Au maldistribution degree and acquired in a
non-destructive manner from the gas sensor element heated to a
predetermined temperature; acquiring a value of the alternative
maldistribution degree index for the inspection target electrode of
the gas sensor element while the gas sensor element is heated to
the predetermined temperature; and determining whether the Au
maldistribution degree satisfies a predetermined standard based on
the calibration curve and the acquired inspection value.
Inventors: |
Okamoto; Taku (Nagoya,
JP), Nakayama; Yuki (Nagoya, JP), Yoshida;
Soichiro (Nagoya, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NGK INSULATORS, LTD. |
Nagoya-shi, Aichi |
N/A |
JP |
|
|
Assignee: |
NGK Insulators, Ltd. (Nagoya,
JP)
|
Family
ID: |
64333187 |
Appl.
No.: |
15/996,560 |
Filed: |
June 4, 2018 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20180356364 A1 |
Dec 13, 2018 |
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Foreign Application Priority Data
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Jun 12, 2017 [JP] |
|
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2017-115093 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N
27/041 (20130101); G01N 27/4163 (20130101); H01L
22/34 (20130101); G01N 27/4175 (20130101); G01N
27/4075 (20130101); G01N 27/4071 (20130101); G01N
27/4076 (20130101); G01N 27/4067 (20130101); G01D
5/2417 (20130101); H01L 2224/131 (20130101); G01D
5/165 (20130101); H01L 2924/0002 (20130101); G01D
5/06 (20130101); G01D 11/245 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
G01R
27/08 (20060101); G01N 27/416 (20060101); G01N
27/04 (20060101); H01L 21/66 (20060101); G01D
5/06 (20060101); G01D 5/241 (20060101); G01D
11/24 (20060101); G01D 5/165 (20060101); G01N
27/406 (20060101); G01N 27/407 (20060101) |
Field of
Search: |
;324/76.11-76.83,459,600,635,644,649,662,671,691,693,699,716 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2016-33510 |
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Mar 2016 |
|
JP |
|
5918434 |
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May 2016 |
|
JP |
|
Primary Examiner: Rios Russo; Raul J
Attorney, Agent or Firm: Mattingly & Malur, PC
Claims
What is claimed is:
1. A method of inspecting an Au (Gold) maldistribution degree at a
noble metal particle surface of an inspection target electrode,
wherein said inspection target electrode is provided in a gas
sensor element made of oxygen-ion conductive solid electrolyte,
said inspection target electrode contains Pt (Platinum) and Au as
noble metal components, said gas sensor element includes a heater
inside, and said Au maldistribution degree is a value defined based
on a ratio of an area of a portion at which Au is exposed on said
noble metal particle surface, and calculated from a result of XPS
(X-ray photoelectron spectroscopy) analysis or AES (Auger electron
spectroscopy) analysis on said inspection target electrode, said
method comprising the steps of: a) producing, in advance, a
calibration curve representing a relation between said Au
maldistribution degree and a predetermined alternative
maldistribution degree index correlated with said Au
maldistribution degree and acquired in a non-destructive manner
from said gas sensor element heated to a predetermined temperature
by said heater; b) acquiring, as an inspection value, a value of
said alternative maldistribution degree index for said inspection
target electrode of the gas sensor element as an inspection target
while the gas sensor element is heated to said predetermined
temperature; and c) determining whether the Au maldistribution
degree at said inspection target electrode satisfies a
predetermined standard based on said calibration curve produced
through said step a) and said inspection value acquired through
said step b).
2. The inspection method according to claim 1, wherein said
alternative maldistribution degree index is reaction resistance
between said inspection target electrode and a predetermined
reference electrode provided in said gas sensor element, the
reaction resistance being obtained by complex impedance
measurement.
3. The inspection method according to claim 1, wherein said
alternative maldistribution degree index is a value of
direct-current resistance between said inspection target electrode
and said reference electrode when predetermined direct-current
voltage is applied between said inspection target electrode and a
predetermined reference electrode provided in said gas sensor
element.
4. The inspection method according to claim 1, wherein said
alternative maldistribution degree index is a value of direct
current flowing between said inspection target electrode and said
reference electrode when predetermined direct-current voltage is
applied between said inspection target electrode and a
predetermined reference electrode provided in said gas sensor
element.
5. The inspection method according to claim 1, wherein said Au
maldistribution degree is defined by an Au surface concentration
that is a ratio of an area of a portion at which said Au is exposed
to an entire area of said noble metal particle surface.
6. The inspection method according to claim 1, wherein said Au
maldistribution degree is defined by an Au abundance ratio that is
a ratio of the area of a portion at which said Au is exposed to an
area of a portion at which Pt is exposed on said noble metal
particle surface.
7. The inspection method according to claim 1, wherein said
predetermined temperature is 640.degree. C. (Celsius) to
850.degree. C.
8. The inspection method according to claim 1, wherein said gas
sensor element is a mixed-potential gas sensor element, said
inspection target electrode is a sensing electrode made of a cermet
of Pt, Au, and zirconia and configured to sense a measurement
target gas component in measurement gas, and said reference
electrode is made of a cermet of Pt and zirconia.
9. The inspection method according to claim 2, wherein said Au
maldistribution degree is defined by an Au surface concentration
that is a ratio of an area of a portion at which said Au is exposed
to an entire area of said noble metal particle surface.
10. The inspection method according to claim 2, wherein said Au
maldistribution degree is defined by an Au abundance ratio that is
a ratio of an area of a portion at which said Au is exposed to an
area of a portion at which Pt is exposed on said noble metal
particle surface.
11. The inspection method according to claim 2, wherein said
predetermined temperature is 640.degree. C. to 850.degree. C.
12. The inspection method according to claim 2, wherein said gas
sensor element is a mixed-potential gas sensor element, said
inspection target electrode is a sensing electrode made of a cermet
of Pt, Au, and zirconia and configured to sense a measurement
target gas component in measurement gas, and said reference
electrode is made of a cermet of Pt and zirconia.
13. The inspection method according to claim 3, wherein said Au
maldistribution degree is defined by an Au surface concentration
that is a ratio of an area of a portion at which said Au is exposed
to an entire area of said noble metal particle surface.
14. The inspection method according to claim 3, wherein said Au
maldistribution degree is defined by an Au abundance ratio that is
a ratio of an area of a portion at which said Au is exposed to an
area of a portion at which Pt is exposed on said noble metal
particle surface.
15. The inspection method according to claim 3, wherein said
predetermined temperature is 640.degree. C. to 850.degree. C.
16. The inspection method according to claim 3, wherein said gas
sensor element is a mixed-potential gas sensor element, said
inspection target electrode is a sensing electrode made of a cermet
of Pt, Au, and zirconia and configured to sense a measurement
target gas component in measurement gas, and said reference
electrode is made of a cermet of Pt and zirconia.
17. The inspection method according to claim 4, wherein said Au
maldistribution degree is defined by an Au surface concentration
that is a ratio of an area of a portion at which said Au is exposed
to an entire area of said noble metal particle surface.
18. The inspection method according to claim 4, wherein said Au
maldistribution degree is defined by an Au abundance ratio that is
a ratio of an area of a portion at which said Au is exposed to an
area of a portion at which Pt is exposed on said noble metal
particle surface.
19. The inspection method according to claim 4, wherein said
predetermined temperature is 640.degree. C. to 850.degree. C.
20. The inspection method according to claim 4, wherein said gas
sensor element is a mixed-potential gas sensor element, said
inspection target electrode is a sensing electrode made of a cermet
of Pt, Au, and zirconia and configured to sense a measurement
target gas component in measurement gas, and said reference
electrode is made of a cermet of Pt and zirconia.
Description
CROSS-REFERENCE TO RELATED APPLICATION
The present application claims priority from Japanese application
JP 2017-115093, filed on Jun. 12, 2017, the content of which is
hereby incorporated by reference into this application.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method of inspecting an
electrode provided in a gas sensor element, and particularly
relates to non-destructive inspection of an Au maldistribution
degree.
Description of the Background Art
Gas sensors configured to sense a predetermined gas component in
measurement gas to determine the concentration thereof come in
various types such as a semiconductor type, a catalytic combustion
type, an oxygen-concentration difference sensing type, a limiting
current type, and a mixed-potential type. Some of these gas sensors
include a sensor element mainly made of ceramic which is solid
electrolyte such as zirconia.
Such gas sensors include publicly known mixed-potential gas sensors
whose sensing target components is hydrocarbon gas or ammonia gas,
having a sensing electrode made of a cermet of noble metals
(specifically, Pt and Au) and oxygen-ion conductive solid
electrolyte on a surface of the sensor element, and ensuring a
sufficient detection sensitivity by maldistribution of Au on the
surface of a noble metal particle forming the sensing electrode (by
increasing the Au abundance ratio at the surface of a noble metal
particle) (refer to Japanese Patent Application Laid-Open No.
2016-33510 and Japanese Patent No. 5918434, for example).
In aspects disclosed in Japanese Patent Application Laid-Open No.
2016-33510 and Japanese Patent No. 5918434, the Au abundance ratio
at the surface of the sensing electrode provided in the sensor
element means the area ratio of a portion covered by Au to a
portion at which Pt is exposed on the surface of a noble metal
particle forming the sensing electrode, and the Au abundance ratio
is evaluated based on a result of X-ray photoelectron spectroscopy
(XPS) analysis or Auger electron spectroscopy (AES) analysis on the
sensing electrode.
In this case, the sensing electrode, which is covered by a surface
protective layer, needs to be exposed to perform the evaluation of
the Au abundance ratio at the sensing electrode. This exposure can
be achieved by, for example, peeling the surface protective layer
or breaking the sensor element at the position of the sensing
electrode to analyze the broken-out section of the sensing
electrode. However, when the sensing electrode has a small film
thickness or a small electrode area, it is difficult to desirably
exposure the sensing electrode for analysis in some cases.
As a matter of course, evaluation of the Au abundance ratio by
peeling the surface protective layer or breaking the sensor element
cannot be used in one-hundred percent inspection in mass production
of the sensor elements.
SUMMARY
The present invention relates to a method of inspecting an
electrode provided in a gas sensor element, and is particularly
directed to non-destructive inspection of an Au maldistribution
degree on a noble metal particle surface of the electrode.
According to the present invention, a method of inspecting an Au
maldistribution degree at a noble metal particle surface of an
inspection target electrode, when the inspection target electrode
is provided in a gas sensor element made of oxygen-ion conductive
solid electrolyte, and the inspection target electrode contains Pt
and Au as noble metal components, and the gas sensor element
includes a heater inside, the method includes the steps of: a)
producing, in advance, a calibration curve representing a relation
between the Au maldistribution degree and a predetermined
alternative maldistribution degree index correlated with the Au
maldistribution degree and acquired in a non-destructive manner
from the gas sensor element heated to a predetermined temperature
by the heater; b) acquiring, as an inspection value, a value of the
alternative maldistribution degree index for the inspection target
electrode of the gas sensor element as an inspection target while
the gas sensor element is heated to the predetermined temperature;
and c) determining whether the Au maldistribution degree at the
inspection target electrode satisfies a predetermined standard
based on the calibration curve produced through the step a) and the
inspection value acquired through the step b).
According to the present invention, the Au maldistribution degree
at an inspection target electrode can be inspected without
destructing a sensor element and faster than a case in which XPS
analysis or AES analysis is performed.
Preferably, the alternative maldistribution degree index is
preferably a value of direct-current resistance between the
inspection target electrode and the reference electrode or a value
of direct current flowing between the inspection target electrode
and the reference electrode when predetermined direct-current
voltage is applied between the inspection target electrode and a
predetermined reference electrode provided in the gas sensor
element.
With this configuration, the Au maldistribution degree at the
inspection target electrode can be inspected faster than a case in
which complex impedance measurement is performed.
The present invention is intended to provide a method capable of
inspecting, by a simple method, the Au maldistribution degree on a
noble metal particle surface of a sensing electrode provided in a
sensor element of a gas sensor.
These and other objects, features, aspects and advantages of the
present invention will become more apparent from the following
detailed description of the present invention when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are sectional pattern diagrams each schematically
illustrating a configuration of a gas sensor 100A;
FIG. 2 is a diagram illustrating the flow of processing of
manufacturing a sensor element 101A;
FIGS. 3A to 3D are schematic Nyquist diagrams for description of
derivation of the reaction resistance of the sensor element
101A;
FIG. 4 is a Nyquist diagram illustrating a result of two-terminal
complex impedance measurement performed at a sensor drive
temperature of 640.degree. C. to obtain the reaction resistances of
five sensor elements 101A having different Au maldistribution
degrees at a sensing electrode 10;
FIG. 5 is a Nyquist diagram illustrating a result of two-terminal
complex impedance measurement performed at a sensor drive
temperature of 750.degree. C. to obtain the reaction resistances of
the five sensor elements 101A having different Au maldistribution
degrees at the sensing electrode 10;
FIG. 6 is a Nyquist diagram illustrating a result of two-terminal
complex impedance measurement performed at a sensor drive
temperature of 850.degree. C. to obtain the reaction resistances of
the five sensor elements 101A having different Au maldistribution
degrees at the sensing electrode 10;
FIG. 7 is a diagram plotting a reaction resistance value per unit
area of the sensing electrode 10 against an Au surface
concentration;
FIG. 8 is a diagram illustrating VI profiles of the five sensor
elements 101A having different Au maldistribution degrees at the
sensing electrode 10 when direct-current voltage is applied between
the sensing electrode 10 and a reference electrode 20 at different
voltage values at a sensor drive temperature of 640.degree. C.;
and
FIG. 9 is a diagram plotting the direct-current resistance value
per unit area of the sensing electrode 10 against the Au surface
concentration.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
<Exemplary Configuration of Gas Sensor>
FIGS. 1A and 1B are sectional pattern diagrams each schematically
illustrating a configuration of a gas sensor 100A as an exemplary
inspection target in an inspection method according to the present
preferred embodiment. FIG. 1A is a vertical sectional view of a
sensor element 101A, which is a main component of the gas sensor
100A, taken along the longitudinal direction of the sensor element
101A. FIG. 1B is a diagram illustrating a section of the sensor
element 101A taken along line A-A' in FIG. 1A perpendicular to the
longitudinal direction of the sensor element 101A. The inspection
method according to the present preferred embodiment generally
relates to an inspection performed on a sensing electrode 10
provided on a surface of the sensor element 101A in the
manufacturing process of the gas sensor 100A.
The gas sensor 100A is what is called a mixed-potential type gas
sensor. Generally, the gas sensor 100A determines the concentration
of a gas component as a measurement target in measurement gas by
utilizing a potential difference generated between the sensing
electrode 10 provided on the surface of the sensor element 101A
mainly made of ceramic as oxygen-ion conductive solid electrolyte
such as zirconia (ZrO.sub.2) and a reference electrode 20 provided
inside the sensor element 101A, the potential difference being
attributable to the difference between the concentrations of the
gas component near the both electrodes based on the principle of
mixed potential.
More specifically, the gas sensor 100A preferably determines the
concentration of a predetermined gas component in measurement gas
that is exhaust gas present in an exhaust pipe of an internal
combustion engine such as a diesel engine or a gasoline engine.
Examples of gas components as measurement targets include
hydrocarbon gas such as C.sub.2H.sub.4, C.sub.3H.sub.6, or n-C8,
carbon monoxide (CO), ammonia (NH.sub.3), steam (H.sub.2O),
nitrogen monoxide (NO), and nitrogen dioxide (NO.sub.2). However,
in the present specification, the hydrocarbon gas also includes
carbon monoxide (CO) in some cases.
When a plurality of kinds of gas components are contained in
measurement gas, the potential difference generated between the
sensing electrode 10 and the reference electrode 20 has a value
including contributions from all of the plurality of kinds of gas
components in principle. However, in the same combinations of the
contained gas components, the concentration values of individual
kinds of gas can be individually obtained by preferably setting the
drive temperature of the sensor element 101A and adjusting the
properties (such as porosity and pore size) of a surface protective
layer 50 to be described later. Alternatively, as for hydrocarbon
gas, the concentrations of a plurality of kinds of hydrocarbon gas
can be directly calculated in some cases. Certainly, it is
acceptable that the gas sensor 100A is used under the condition
that a gas component contained in measurement gas is limited to a
particular gas component in advance and the concentration of the
gas component is obtained.
In addition to the sensing electrode 10 and the reference electrode
20 described above, the sensor element 101A mainly includes a
reference gas introduction layer 30, a reference gas introduction
space 40, and the surface protective layer 50.
The sensor element 101A has a structure in which six layers, i.e.,
a first solid electrolyte layer 1, a second solid electrolyte layer
2, a third solid electrolyte layer 3, a fourth solid electrolyte
layer 4, a fifth solid electrolyte layer 5, and a sixth solid
electrolyte layer 6 each made of oxygen-ion conductive solid
electrolyte are stacked in this order from the bottom in FIGS. 1A
and 1B. The sensor element 101A additionally includes other
components mainly between the layers or on an outer peripheral
surface of the element. The solid electrolyte of which the six
layers are made is fully airtight.
However, it is not essential that the gas sensor 100A include the
sensor element 101A as such a six-layer laminated body. The sensor
element 101A may be formed as a laminated body having a larger or
smaller number of layers or may not have a laminated structure.
In the following description, for sake of convenience, a surface
located above the sixth solid electrolyte layer 6 in FIGS. 1A and
1B is referred to as a front surface Sa of the sensor element 101A,
and a surface located below the first solid electrolyte layer 1 in
FIGS. 1A and 1B is referred to as a rear surface Sb of the sensor
element 101A. When the concentration of a predetermined gas
component in measurement gas is obtained by using the gas sensor
100A, a predetermined range extending from a distal end E1 at one
end of the sensor element 101A and including at least the sensing
electrode 10 is disposed in a measurement gas atmosphere, while the
other portion including a base end E2 at the other end is disposed
avoiding contact with the measurement gas atmosphere.
The sensing electrode 10 is an electrode for sensing measurement
gas. The sensing electrode 10 is formed as a porous cermet
electrode made of Pt containing Au at a predetermined ratio, in
other words, a Pt--Au alloy, and zirconia. The sensing electrode 10
is provided in a substantially rectangular shape in plan view at a
position closer to the distal end E1 that is one end in the
longitudinal direction on the front surface Sa of the sensor
element 101A. When the gas sensor 100A is used, a portion of the
sensor element 101A extending at least to a portion in which the
sensing electrode 10 is provided is exposed to measurement gas.
The catalytic activity of the sensing electrode 10 against a
measurement gas component in measurement gas is disabled in a
predetermined concentration range by preferably determining the
composition of the Pt--Au alloy which constitutes the sensing
electrode 10. In other words, the combustion reaction of a
measurement target gas component at the sensing electrode 10 is
prevented or reduced. Accordingly, in the gas sensor 100A, the
potential of the sensing electrode 10 selectively varies in
accordance with (has correlation with) the concentration of a
measurement target gas component through electrochemical reaction.
In other words, the potential of the sensing electrode 10 has
characteristics of high concentration dependency on a measurement
target gas component in a predetermined concentration range and low
concentration dependency on any other component in measurement
gas.
More specifically, in the sensor element 101A of the gas sensor
100A according to the present preferred embodiment, Au is
concentrated on the surface of a Pt--Au alloy particle forming the
sensing electrode 10. In other words, an Au abundance ratio, which
is the area ratio of a portion covered by Au to a portion at which
Pt is exposed on the surface of a noble metal (Pt--Au alloy)
particle forming the sensing electrode 10, is increased.
Accordingly, the potential of the sensing electrode 10 exhibits
significant dependency on the concentration of a measurement target
gas component in a predetermined concentration range.
As disclosed in Japanese Patent Application Laid-Open No.
2016-33510, the Au abundance ratio can be calculated from peak
intensities of Au and Pt at detection peaks, which can be obtained
by X-ray photoelectron spectroscopy (XPS), by using a relative
sensitivity coefficient method. Alternatively, as disclosed in
Japanese Patent No. 5918434, the Au abundance ratio can be
calculated by using detected values of Au and Pt in an Auger
spectrum, which can be calculated by performing Auger electron
spectroscopy (AES) analysis on a surface of a noble metal
particle.
The Au abundance ratio increases as the degree of concentration of
Au (Au maldistribution degree) on the surface of a noble metal
particle of the sensing electrode 10 increases.
A high Au maldistribution degree at the sensing electrode 10 means
that the concentration of Au on the surface (Au surface
concentration) of a noble metal particle of the sensing electrode
10 is high. The Au abundance ratio indicates the ratio of an area
of a portion covered by Au to an area of a portion at which Pt is
exposed on the surface of a noble metal particle, whereas the Au
surface concentration corresponds to the ratio of an area of a
portion at which Au is exposed to an area of a whole surface of a
noble metal particle. In place of or together with calculation of
the Au abundance ratio, the Au surface concentration can be
calculated by using a result of the XPS (X-ray photoelectron
spectroscopy) analysis or the AES (Auger electron spectroscopy)
analysis described above for calculation of the Au abundance ratio.
In addition, the Au abundance ratio and the Au surface
concentration have a mutually convertible relation. This is
because, when S.sub.Au and S.sub.Pt represent the areas of portions
at which Au and Pt, respectively, are exposed on a surface of a
noble metal particle, the Au abundance ratio is given by
S.sub.Au/S.sub.Pt, and the Au surface concentration (%) is given by
100.times.S.sub.Au/(S.sub.Au+S.sup.Pt).
For example, when the area of the portion at which Pt is exposed is
equal to the area of the portion covered by Au, in other words,
when S.sub.Au=S.sub.Pt, the Au abundance ratio is one, and the Au
surface concentration is 50%.
Thus, when a threshold is appropriately set, the Au surface
concentration can be used as an index of the Au maldistribution
degree in place of the Au abundance ratio.
The reference electrode 20 has a substantially rectangular shape in
plan view and is provided inside the sensor element 101A and serves
as a reference when the concentration of measurement gas is
obtained. The reference electrode 20 is formed as a porous cermet
electrode made of Pt and zirconia.
The reference electrode 20 is enough to be formed to have a
porosity of 10% or more and 30% or less and a thickness of 5 .mu.m
or more and 15 .mu.m or less. As illustrated in FIGS. 1A and 1B,
the reference electrode 20 may have a plane size smaller than or
equal to that of the sensing electrode 10.
The reference gas introduction layer 30 is made of porous alumina
and provided to cover the reference electrode 20 inside the sensor
element 101A. The reference gas introduction space 40 is an
internal space provided near the base end E2 of the sensor element
101A. Air (oxygen), serving as a reference gas in the determination
of the concentration of an inspection target gas component, is
externally introduced to the reference gas introduction space
40.
The reference gas introduction space 40 and the reference gas
introduction layer 30 are communicated with each other, so that the
surrounding of the reference electrode 20 is constantly filled with
air (oxygen) through the reference gas introduction space 40 and
the reference gas introduction layer 30 when the gas sensor 100A is
used. Thus, during the use of the gas sensor 100A, the reference
electrode 20 always has a constant potential.
The reference gas introduction space 40 and the reference gas
introduction layer 30 are prevented from contacting with
measurement gas by the surrounding solid electrolyte. Thus, the
reference electrode 20 does not come into contact with the
measurement gas even when the sensing electrode 10 is exposed to
the measurement gas.
In the configuration illustrated in FIG. 1A, the reference gas
introduction space 40 is provided in a manner that a part of the
fifth solid electrolyte layer 5 is communicated with the outside on
the base end E2 side of the sensor element 101A. The reference gas
introduction layer 30 is provided between the fifth solid
electrolyte layer 5 and the sixth solid electrolyte layer 6 so as
to extend in the longitudinal direction of the sensor element 101A.
The reference electrode 20 is provided at a position below the
center of gravity of the sensing electrode 10 in FIGS. 1A and
1B.
The surface protective layer 50 is a porous layer made of alumina
and covering at least the sensing electrode 10 on the front surface
Sa of the sensor element 101A. The surface protective layer 50 is
provided as an electrode protective layer that prevents or reduces
degradation of the sensing electrode 10 due to continuous exposure
to measurement gas when the gas sensor 100A is used. In the
configuration illustrated in FIG. 1A, the surface protective layer
50 covers not only the sensing electrode 10 but also a
substantially entire portion of the front surface Sa of the sensor
element 101A except for a predetermined range from the distal end
E1.
As illustrated in FIG. 1B, the gas sensor 100A is provided with a
potentiometer 60 capable of measuring the potential difference
between the sensing electrode 10 and the reference electrode 20.
FIG. 1B schematically illustrates wiring between the potentiometer
60 and each of the sensing electrode 10 and the reference electrode
20. However, in the actual sensor element 101A, a connection
terminal (not illustrated) corresponding to each electrode is
provided on the front surface Sa or the rear surface Sb on the base
end E2 side, and a wiring pattern (not illustrated) connecting each
electrode and the corresponding connection terminal is formed on
the front surface Sa and inside the element. The sensing electrode
10 and the reference electrode 20 are each electrically connected
with the potentiometer 60 through the wiring pattern and the
connection terminal. Hereinafter, the potential difference between
the sensing electrode 10 and the reference electrode 20, which is
measured by the potentiometer 60 is also referred to as a sensor
output.
The sensor element 101A further includes a heater part 70
configured to perform temperature adjustment involving heating and
temperature maintenance of the sensor element 101A to increase the
oxygen-ion conductivity of the solid electrolyte. The heater part
70 includes a heater electrode 71, a heater 72, a through-hole 73,
a heater insulating layer 74, and a pressure diffusion hole 75.
The heater electrode 71 is formed in contact with the rear surface
Sb of the sensor element 101A (a lower surface of the first solid
electrolyte layer 1 in FIGS. 1A and 1B). Power can be supplied to
the heater part 70 from the outside when the heater electrode 71 is
connected with an external power source (not illustrated).
The heater 72 is an electric resistor provided inside the sensor
element 101A. The heater 72 is connected with the heater electrode
71 through the through-hole 73, and generates heat with being
powered externally through the heater electrode 71 to perform
heating and temperature maintenance of the solid electrolyte
forming the sensor element 101A.
In the configuration illustrated in FIGS. 1A and 1B, the heater 72
is buried while being vertically sandwiched between the second
solid electrolyte layer 2 and the third solid electrolyte layer 3
in a range extending from the base end E2 to a position below the
sensing electrode 10 near the distal end E1. With this
configuration, the entire sensor element 101A can be adjusted to a
temperature at which the solid electrolyte is activated.
The heater insulating layer 74 is made of insulator such as alumina
and formed on upper and lower surfaces of the heater 72. The heater
insulating layer 74 is formed to provide electric insulation
between the second solid electrolyte layer 2 and the heater 72 and
electric insulation between the third solid electrolyte layer 3 and
the heater 72.
The pressure diffusion hole 75 is a site penetrating through the
third solid electrolyte layer 3 and the fourth solid electrolyte
layer 4 and communicated with the reference gas introduction space
40. The pressure diffusion hole 75 is formed to reduce increase in
internal pressure along with increase in temperature inside the
heater insulating layer 74.
When the concentration of a target gas component included in
measurement gas is obtained by using the gas sensor 100A having the
above-described configuration, only the predetermined range of the
sensor element 101A extending from the distal end E1 and including
at least the sensing electrode 10 is disposed in a space in which
the measurement gas is present as described above, while the base
end E2 side is isolated from the space to supply air (oxygen) into
the reference gas introduction space 40. The heater 72 heats the
sensor element 101A to an appropriate temperature of 400.degree. C.
to 800.degree. C., preferably 500.degree. C. to 700.degree. C.,
more preferably 500.degree. C. to 600.degree. C. The temperature at
which the heater 72 heats the sensor element 101A is also referred
to as a sensor drive temperature.
In this state, a potential difference is generated between the
sensing electrode 10 exposed to the measurement gas and the
reference electrode 20 disposed in the air. However, as described
above, the potential of the sensing electrode 10 selectively has
concentration dependency on an inspection target gas component in
the measurement gas whereas the potential of the reference
electrode 20 being disposed in an air (with constant oxygen
concentration) atmosphere is maintained constant. Thus, the
potential difference (sensor output) substantially has a value in
accordance with the composition of the measurement gas present in
the surrounding of the sensing electrode 10. Accordingly, a
constant functional relation (referred to as a sensitivity
characteristic) holds between the concentration of the inspection
target gas component and the sensor output. Hereinafter, such a
sensitivity characteristic is also referred to as the sensitivity
characteristic of the sensing electrode 10.
When actually obtaining the concentration of an inspection target
gas component, the sensitivity characteristic is experimentally
specified in advance by measuring the sensor output for each
measurement gas of a plurality of mutually different mixed gases,
in which the concentration of each inspection target gas component
is known. Accordingly, when the gas sensor 100A is actually used,
the sensor output, which momentarily changes in accordance with the
concentration of an inspection target gas component in measurement
gas, is converted into the concentration of the inspection target
gas component based on the sensitivity characteristic by an
arithmetic processing unit (not illustrated). In this manner, the
concentration of the inspection target gas component in the
measurement gas can be obtained substantially in real time.
<Manufacturing Process of Sensor Element>
The sensor element 101A having the layer structure as illustrated
in FIGS. 1A and 1B can be manufactured through manufacturing
processes disclosed in, for example, Japanese Patent Application
Laid-Open No. 2016-33510 and Japanese Patent No. 5918434.
Generally, the sensor element 101A having the above-described
configuration is manufactured as follows. First, predetermined
processing, printing of circuit patterns for electrodes, and the
like are performed on a plurality of ceramic green sheets
containing oxygen-ion conductive solid electrolyte (for example,
yttrium partially stabilized zirconia (YSZ)) as a ceramic component
and corresponding to the respective solid electrolyte layers.
Thereafter, the ceramic green sheets are laminated in a
predetermined order, and a laminated body thus obtained is cut into
units of elements to obtain a plurality of element bodies. Then,
the element bodies are simultaneously fired to achieve integration
of each element body, thereby simultaneously manufacturing a
plurality of sensor elements 101A.
FIG. 2 is a diagram illustrating the flow of processing of
manufacturing the sensor element 101A, which is showed for the
purpose of confirmation. When the sensor element 101A is
manufactured, first, a blank sheet (not illustrated), which is a
green sheet on which no pattern is formed, is prepared (step S1).
Specifically, six blank sheets corresponding to the first solid
electrolyte layer 1 to the sixth solid electrolyte layer 6 are
prepared. Each blank sheet is provided with a plurality of sheet
holes used for positioning at printing and laminating. The sheet
holes are formed in advance through, for example, punching
processing by a punching device. Green sheets corresponding to
layers forming an internal space also include penetrating portions
corresponding to the internal space in advance through, for
example, the punching processing as described above. Not all blank
sheets corresponding to the respective layers of the sensor element
101A need to have equal thicknesses.
After the blank sheets corresponding to the respective layers are
prepared, pattern printing and dry processing are performed to form
various kinds of patterns on each blank sheet (step S2).
Specifically, for example, a pattern of each electrode, a pattern
of the heater 72, and an internal wire (not illustrated) are
formed. In addition, a pattern of the surface protective layer 50
may be printed.
The printing of each pattern is performed by applying, to a blank
sheet, pattern formation paste prepared in accordance with a
characteristic requested for each formation target by using a
well-known screen printing technique. Well-known drying means can
be used for the dry processing after the printing.
After the pattern printing is completed, printing of bonding paste
and dry processing are performed (step S3). The bonding paste is
used to laminate and bond the green sheets corresponding to the
respective layers to each other. A well-known screen printing
technique can be used for the printing of the bonding paste, and
well-known dry means can be used for the dry processing after the
printing.
Subsequently, crimping processing is performed (step S4). In the
crimping processing, the green sheets to which an adhesive has been
applied are stacked in a predetermined order, and the stacked green
sheets are crimped under predetermined temperature and pressure
conditions to thereby form a laminated body. Specifically, crimping
is performed by stacking and holding the green sheets as a target
of lamination in a predetermined lamination jig (not illustrated)
while positioning the green sheets at the sheet holes, and then
heating and pressurizing the green sheets together with the
lamination jig using a lamination machine, such as a known
hydraulic pressing machine. The pressure, temperature, and time for
heating and pressurizing depend on a lamination machine to be used,
and these conditions may be set appropriately to achieve good
lamination. The surface protective layer 50 may be formed on the
laminated body as obtained.
After the laminated body is obtained as described above, the
laminated body is cut out at a plurality of positions to obtain a
plurality of element bodies (step S5). The cut out element bodies
are fired under predetermined conditions, thereby producing the
sensor elements 101A as described above (step S6). This means that
the sensor element 101A is produced by integral firing (co-firing)
of the solid electrolyte layers and the electrodes. The firing
temperature is preferably 1,200.degree. C. or higher and
1,500.degree. C. or lower (e.g., 1,400.degree. C.). Integral firing
performed in such a manner provides sufficient adhesion strength to
each of the electrodes of the sensor element 101A. This contributes
to improvement in durability of the sensor element 101A.
The sensor element 101A obtained in this manner is subjected to
various kinds of inspection processes such as a characteristic
inspection, an appearance inspection, and a strength inspection.
Only the sensor element 101A having passed all inspection processes
is housed in a predetermined housing and incorporated in a main
body (not illustrated) of the gas sensor 100A.
The pattern formation paste (conductive paste) used to form the
sensing electrode 10 can be produced by preparing an Au
ion-containing liquid as an Au starting material and mixing the Au
ion-containing liquid with powdered Pt, powdered zirconia, and a
binder. Any binder may be appropriately selected, as long as it can
disperse any other raw material to an extent appropriate for
printing and is burned out by firing.
The Au ion-containing liquid is obtained by dissolving a salt
containing an Au ion or an organometallic complex containing an Au
ion in a solvent. The Au ion-containing salt may be, for example,
tetrachloroauric(III) acid (HAuCl.sub.4), sodium chloroaurate(III)
(NaAuCl.sub.4), or potassium dicyanoaurate(I) (KAu(CN).sub.2). The
Au ion-containing organometallic complex may be, for example,
gold(III) diethylenediamine trichloride ([Au(en).sub.2]Cl.sub.3),
gold(III) dichloro(1,10-phenanthroline)chloride
([Au(phen)Cl.sub.2]Cl), dimethyl(trifluoroacetylacetonate)gold, or
dimethyl(hexafluoroacetylacetonate)gold. Tetrachloroauric(III) acid
or gold(III) diethylenediamine chloride ([Au(en).sub.2]Cl.sub.3) is
preferably used from the viewpoint of no impurity such as Na or K
remaining in the electrode, easy handling, or dissolvability in the
solvent. The solvent may be acetone, acetonitrile, or formamide as
well as alcohols such as methanol, ethanol, and propanol.
Mixing can be performed by well-known means such as instillation.
Although the obtained conductive paste contains Au present in ionic
(complex ionic) state, the sensing electrode 10 formed in the
sensor element 101A obtained through the above-mentioned
manufacturing process contain Au mainly as an elemental substrate
or an alloy with Pt.
Alternatively, the conductive paste for the sensing electrode 10
may be prepared by using coated powder, which is obtained by
coating powdered Pt with Au, as an Au starting raw material,
instead of preparing the paste through liquid-state Au mixing as
described above. In such a case, a conductive paste for the outer
pump electrode is prepared by mixing the coated powder, zirconia
powder, and a binder. Here, the coated powder may be obtained by
covering the particle surface of powdered Pt with an Au film or
applying Au particles to Pt powder particles.
<Inspection of Au Maldistribution Degree of Sensing
Electrode>
The following describes inspection of the Au maldistribution degree
at the sensing electrode 10 provided on the surface of the sensor
element 101A of the gas sensor 100A manufactured through the
above-described manufacturing process. The inspection is performed
as one of the above-described various kinds of inspection
processes.
As described above, a plurality of sensor elements 101A are
manufactured at once by simultaneously firing a plurality of
element bodies cut out from one laminated body. Thus, a plurality
of sensor elements 101A obtained from one laminated body or a
plurality of sensor elements 101A obtained by firing, under an
identical condition, a plurality of element bodies obtained from a
plurality of laminated bodies manufactured under an identical
manufacturing condition are required to ideally have identical
characteristics. Any variance in the characteristics is required to
be within the range of a predetermined standard (inspection
standard).
The same applies to the Au maldistribution degree of the sensing
electrode 10, which largely affects the sensitivity characteristic
of the sensor element 101A. Thus, at industrial mass production of
the sensor element 101A, the Au maldistribution degree at the
sensing electrode 10 of each sensor element 101A is required to be
within the range of a predetermined standard (inspection
standard).
As described above, direct evaluation of the Au maldistribution
degree need to be performed based on the Au abundance ratio or the
Au surface concentration calculated from results of analysis by XPS
or AES. However, as a matter of course, the sensor element 101A
cannot be inspected in a destructive manner in a mass production
process. Meanwhile, a non-destructive inspection is limited to a
case in which the sensor element 101A does not include the surface
protective layer 50 or a case in which the surface protective layer
50 is formed after inspection, which is not a versatile method.
In the inspection method according to the present preferred
embodiment, in place of direct evaluation of the Au maldistribution
degree at the sensing electrode 10 based on an XPS or AES
measurement result, the Au maldistribution degree is evaluated by
using, as an alternative evaluation index (alternative
maldistribution degree index) of the Au maldistribution degree, a
physical property value correlated with the Au maldistribution
degree. Specifically, the evaluation is performed in two aspects
exemplarily described below with different physical property values
actually used as the alternative maldistribution degree index.
(First Aspect: Evaluation Based on Reaction Resistance)
In the present aspect, reaction resistance (electrode reaction
resistance) between the sensing electrode 10 and the reference
electrode 20, which is correlated with the Au abundance ratio or
the Au surface concentration is used as the alternative
maldistribution degree index in the manufacturing process (mass
production process) of the sensor element 101A. Hereinafter, the
reaction resistance between the sensing electrode 10 and the
reference electrode 20 of the sensor element 101A is also simply
referred to as the reaction resistance of the sensor element
101A.
The reaction resistance of the sensor element 101A is obtained from
plotting of a result of two-terminal complex impedance measurement
in a Nyquist diagram having the real axis (Z' axis in units of
.OMEGA.) as the horizontal axis and the imaginary axis (Z'' axis in
units of .OMEGA.) as the vertical axis. The two-terminal complex
impedance measurement is performed by applying alternating-current
voltage at different frequencies between the sensing electrode 10
and the reference electrode 20. FIGS. 3A to 3D are schematic
Nyquist diagrams for description of derivation of the reaction
resistance of the sensor element 101A.
Plotting of measured data obtained by the two-terminal complex
impedance measurement described above ideally obtains a
semicircular curved line C1 starting at a point (Z', Z'')=(R1, 0)
on the real axis as illustrated in FIG. 3A. When the Z' coordinate
value of an end point opposite to (Z', Z'')=(R1, 0) on the curved
line C1 is expressed as R1+R2, the reaction resistance is an
increased value R2 of the Z' coordinate value from R1. The value R1
is an IR resistance (insulation resistance), and corresponds to,
for example, the material resistance of a solid electrolyte forming
a sensor element in a mixed-potential gas sensor such as the gas
sensor 100A. Thus, the value of R1, not R2, varies when anomaly
occurs in the solid electrolyte.
However, the plotting of measured data of the two-terminal complex
impedance measurement does not necessarily draw a semicircle like
the curved line C1 illustrated in FIG. 3A. For example, a result of
the plotting obtains a semicircular curved line C2 illustrated in
FIG. 3B, which starts at an end point (Z', Z'')=(R1, 0) while the
other end point does not reach the real axis Z' but ends at a
halfway point P1, or another result of the plotting obtains a
curved line C3 illustrated in FIG. 3C, which is not in a
semicircular shape but in an arc shape ending at the halfway point
P1.
In these cases, the reaction resistance R2 can be determined by
using the Z' coordinate value of a point of extrapolation from the
point P1 to the real axis Z'.
Another result of the plotting obtains two arcs connected with each
other at a point P2 like a curved line C4 illustrated in FIG. 3D.
In such a case, the arc formed in a range in which the Z' axis
coordinate value is larger than that of the point P2 is reflected
on diffusion resistance in the sensor element 101A. The reaction
resistance can be obtained by extrapolation of the point P2,
similarly to the cases in FIGS. 3B and 3C.
FIGS. 4, 5, and 6 are Nyquist diagrams (Cole-Cole plots)
illustrating results of the two-terminal complex impedance
measurement performed to obtain the reaction resistance for five
sensor elements 101A having different Au maldistribution degrees at
the sensing electrode 10.
More specifically, the sensor drive temperature was set to three
different levels of 640.degree. C., 750.degree. C., and 850.degree.
C. FIGS. 4, 5, and 6 illustrate results obtained when the sensor
drive temperature was 640.degree. C., 750.degree. C., and
850.degree. C., respectively. The surface protective layer 50 was
not formed in any of the five sensor elements 101A to allow
evaluation of the Au maldistribution degree based on XPS
measurement. After the XPS measurement, the two-terminal complex
impedance measurement was performed at each sensor drive
temperature. The measurement was performed under air atmosphere by
using a complex impedance measurement device Versa STAT 4
(manufactured by AMETEK Inc.), while the sensing electrode 10 was
connected to a WE/SE line and the reference electrode 20 was
connected to a CE/RE line. The frequency of alternating-current
voltage was 1 MHz to 0.1 Hz, DC bias voltage was 0 V, and
alternating-current amplitude was 20 mV.
Table 1 shows values of the Au surface concentration (in units of
%), which are calculated based on results of the XPS measurement
for the five sensor elements 101A, and the reaction resistance (in
units of .OMEGA.) at each sensor drive temperature.
TABLE-US-00001 TABLE 1 Au surface Reaction resistance (.OMEGA.)
concentration (%) 640.degree. C. 750.degree. C. 850.degree. C. 0
7219 1038 226 10 10486 1223 256 24 20897 3115 842 36 31459 4673
1068 48 40653 5388 1693
FIG. 7 is a diagram plotting, for each sensor drive temperature,
the value of "reaction resistance/electrode area" (in units of
.OMEGA./mm.sup.2), which is a reaction resistance value per unit
area of the sensing electrode 10, against the Au surface
concentration. The value of "reaction resistance/electrode area"
can be obtained by normalizing the values of the reaction
resistance shown in Table 1 with the area of the sensing
electrode.
FIG. 7 also illustrates, for the plotting result at each sensor
drive temperature, an approximate straight line obtained by least
square approximation when the Au surface concentration is taken to
be x and the "reaction resistance/electrode area" is taken to be y.
The function of each approximate straight line and a determination
coefficient R.sup.2 as the squared value of a correlation
coefficient R are shown below. The area of the sensing electrode 10
is 0.4 mm.sup.2. 640.degree. C.: y=1806.3x+17729, R.sup.2=0.9859;
750.degree. C.: y=250.69x+1803, R.sup.2=0.9684; 850.degree. C.:
y=77.271x+218.5, R.sup.2=0.9522.
As understood from FIG. 7 and the above-described values of the
determination coefficient R.sup.2, the (normalized) reaction
resistance and the Au surface concentration have a linear relation
(strong positive correlation) therebetween in any of the cases of
640.degree. C., 750.degree. C., and 850.degree. C. The same result
can be obtained at any other sensor drive temperature at least in
the temperature range of 640.degree. C. to 850.degree. C. As a
matter of course, when the reaction resistance and the Au surface
concentration have a linear relation therebetween, the reaction
resistance and the Au abundance ratio also have a linear relation
therebetween. Although the reaction resistance value is normalized
with the area of the sensing electrode 10 in the above description,
a calibration curve may be produced based on the linear relation
between the reaction resistance and the Au surface concentration or
the Au abundance ratio without normalization because the area of
the sensing electrode 10 is typically the same between the sensor
elements 101A manufactured under the same condition.
In the present aspect, as the utilization of the fact that such a
linear relation is established between the reaction resistance and
the Au maldistribution degree at the sensing electrode 10, the Au
maldistribution degree at the sensing electrode 10 of the sensor
element 101A is inspected by using the reaction resistance as the
alternative maldistribution degree index through an inspection
process in the manufacturing process of the sensor element
101A.
To achieve this, such preparation is performed that the linear
relation between the reaction resistance and the Au surface
concentration or the Au abundance ratio, as illustrated in FIG. 7,
for the sensor element 101A manufactured under a predetermined
manufacturing condition is specified for a predetermined sensor
drive temperature selected from, for example, the range of
640.degree. C. to 850.degree. C., and is recorded as a calibration
curve in advance. This production of a calibration curve can be
performed by manufacturing a plurality of sensor elements 101A all
satisfying the same manufacturing condition except for different Au
maldistribution degrees, performing the complex impedance
measurement on the sensor elements 101A at a predetermined sensor
drive temperature to measure the reaction resistances thereof, and
then performing XPS or AES measurement to determine the Au surface
concentration or the Au abundance ratio. When the surface
protective layer 50 is provided, the sensing electrode 10 may be
exposed by peeling the surface protective layer 50 or breaking the
element before the XPS or AES measurement.
In an actual inspection process, the reaction resistance of the
sensor element 101A manufactured under the same condition as the
above predetermined manufacturing condition is measured while the
sensor element 101A is driven at the same sensor drive temperature
as that at which a calibration curve is produced, and the obtained
measurement value is acquired as an inspection value. Then, the Au
surface concentration or the Au abundance ratio is determined by
comparing the inspection value with the calibration curve recorded
in advance. When the Au surface concentration or the Au abundance
ratio satisfies an inspection standard set in advance, it is
determined that the sensor element 101A as an inspection target has
passed the inspection.
A specific inspection standard for the Au surface concentration or
the Au abundance ratio may be set in various manners based on, for
example, a measurement target gas component and a measurement
target concentration range (range in which highly sensitive
measurement is desired) of the sensor element 101A as an inspection
target. This is because, depending on the kind of gas, a
concentration range in which highly sensitive measurement can be
performed may differ depending on the Au maldistribution
degree.
Alternatively, the range of the reaction resistance when the Au
surface concentration or the Au abundance ratio satisfies an
inspection standard set in advance may be specified in advance
based on the linear relation between the reaction resistance and
the Au surface concentration or the Au abundance ratio as
illustrated in FIG. 7, and it may be determined that the sensor
element 101A as an inspection target has passed the inspection when
the measurement value (inspection value) of the reaction resistance
belongs to the range.
(Second Aspect: Evaluation Based on Direct-Current Resistance)
The inspection according to the above-described first aspect needs
to perform the complex impedance measurement at different
frequencies to obtain the reaction resistance, and thus takes time.
In the present aspect, to more easily perform inspection of the Au
maldistribution degree at the sensing electrode 10 in a shorter
time than in the first aspect, the direct-current resistance
between the sensing electrode 10 and the reference electrode 20 is
used as the alternative maldistribution degree index when the Au
maldistribution degree is evaluated.
FIG. 8 is a diagram illustrating, for each of the five sensor
elements 101A having different Au maldistribution degrees
(different Au surface concentrations) at the sensing electrode 10,
which are used to obtain the reaction resistance in the first
aspect, a change (V-I profile) of a measurement value of current
flowing between the sensing electrode 10 and the reference
electrode 20 to the applied voltage value, in the case that
direct-current voltage was applied at different voltage values
while the sensor drive temperature was set to 640.degree. C.
Measurement was performed under air atmosphere by using the complex
impedance measurement device Versa STAT 4 (manufactured by AMETEK
Inc.) used in the complex impedance measurement in the first
aspect, while the sensing electrode 10 was connected to a WE/SE
line and the reference electrode 20 was connected to a CE/RE line.
The applied voltage value was differed between 0 V to -1 V.
As understood from FIG. 8, the V-I profile differs in accordance
with the Au maldistribution degree at the sensing electrode 10.
Generally, at the same application voltage, the current value tends
to be larger for the sensor element 101A having a smaller Au
maldistribution degree.
Table 2 shows the Au surface concentration (in units of %), the
(direct-current) current value (in units of A) when the applied
voltage value is -1 V, and the (direct-current) resistance value
(in units of .OMEGA.) for each of the five sensor elements 101A.
The resistance value is obtained by dividing the applied voltage
value (-1 V) by each current value.
TABLE-US-00002 TABLE 2 Au surface concentration (%) Current
(.times.10.sup.-5 A) Resistance (.OMEGA.) 0 -9.47 10560 10 -6.77
14776 24 -2.88 34678 36 -2.27 44144 48 -1.54 64934
FIG. 9 is a diagram plotting, against the Au surface concentration,
the value of "resistance/electrode area" (in units of
.OMEGA./mm.sup.2), which is the value of direct-current resistance
per unit area of the sensing electrode 10 and obtained by
normalizing the resistance value shown in Table 2 with the area of
the sensing electrode.
FIG. 9 also illustrates, for the plotting result, an approximate
straight line calculated by least square approximation when the Au
surface concentration is taken to be x and the "reaction
resistance/electrode area" is taken to be y. The function of the
approximate straight line and the determination coefficient R.sup.2
as the squared value of the correlation coefficient R are shown
below. The area of the sensing electrode 10 is 0.4 mm.sup.2.
y=2842.5x+17464, R.sup.2=0.974.
As understood from FIG. 9 and the above-described value of the
determination coefficient R.sup.2, the (normalized) direct-current
resistance value and the Au surface concentration have a linear
relation (strong positive correlation) therebetween. The same
result can be obtained at any other sensor drive temperature at
least in the temperature range of 640.degree. C. to 850.degree. C.
As a matter of course, when the reaction resistance and the Au
surface concentration have a linear relation therebetween, the
reaction resistance and the Au abundance ratio also have a linear
relation therebetween. Although the reaction resistance value is
normalized with the area of the sensing electrode 10 in the above
description, a calibration curve may be produced based on the
linear relation between the reaction resistance and the Au surface
concentration or the Au abundance ratio without normalization
because the area of the sensing electrode 10 is typically the same
between the sensor elements 101A manufactured under the same
condition.
Thus, as the utilization the fact that such a linear relation is
established, the Au maldistribution degree at the sensing electrode
10 of the sensor element 101A can be inspected by using the value
of direct-current resistance between the sensing electrode 10 and
the reference electrode 20 as the alternative maldistribution
degree index.
To achieve this, such preparation is performed that the linear
relation between the value of direct-current resistance between the
sensing electrode 10 and the reference electrode 20 and the Au
surface concentration or the Au abundance ratio, as illustrated in
FIG. 9, for the sensor element 101A manufactured under a
predetermined manufacturing condition is specified for a
predetermined sensor drive temperature selected from, for example,
the range of 640.degree. C. to 850.degree. C. and a predetermined
direct-current voltage value, and is recorded as a calibration
curve in advance. Specifically, this production of a calibration
curve can be performed by manufacturing a plurality of sensor
elements 101A all satisfying the same manufacturing condition
except for different Au maldistribution degrees, measuring the
direct current value by applying direct-current voltage at a
predetermined voltage value (for example, -1 V) between the sensing
electrode 10 and the reference electrode 20 of each sensor element
101A while being driven at a predetermined sensor drive
temperature, thereby obtaining the direct-current resistance value,
and then performing XPS or AES measurement to determine the Au
surface concentration or the Au abundance ratio. Also in the
present aspect, when the surface protective layer 50 is provided,
the sensing electrode 10 may be exposed by peeling the surface
protective layer 50 or breaking the element before the XPS or AES
measurement.
In an actual inspection process, the value of direct current
between the sensing electrode 10 and the reference electrode 20 in
the sensor element 101A manufactured under a condition same as the
above predetermined manufacturing condition is measured at a sensor
drive temperature and a direct-current voltage value same as those
at which a calibration curve is produced, and the value of
direct-current resistance calculated from the obtained measurement
value is acquired as an inspection value. Then, the Au surface
concentration or the Au abundance ratio is determined by comparing
the inspection value with the calibration curve recorded in
advance. When the Au surface concentration or the Au abundance
ratio satisfies an inspection standard set in advance, it is
determined that the sensor element 101A as an inspection target has
passed the inspection.
In this second aspect, measurement performed on the sensor elements
101A as individual inspection targets in the inspection process
only involves single current measurement with application of a
predetermined direct-current voltage value (for example, -1 V).
Thus, in the second aspect, inspection can be performed faster than
in the first aspect in which measurement needs to be repeated with
different frequencies of alternating-current voltage to obtain the
reaction resistance value.
Alternatively, since the linear relation between the value of
direct-current resistance and the Au surface concentration or the
Au abundance ratio as illustrated in FIG. 9 is obtained under a
condition that the applied voltage value is constant, the range of
the current value when the Au surface concentration or the Au
abundance ratio satisfies an inspection standard set in advance may
be specified in advance, and it may be determined that the sensor
element 101A as an inspection target has passed inspection when a
measured current value belongs to the range. In this case, the
current value is an inspection value.
In any of the inspection methods according to the first and second
aspects described above, the Au maldistribution degree at the
sensing electrode 10 can be inspected without destructing the
sensor elements 101A, except for that used to produce a calibration
curve, by using the alternative maldistribution degree index
correlated with the Au abundance ratio or the Au surface
concentration, which indicates the Au maldistribution degree.
Accordingly, it can be determined fast whether the Au
maldistribution degree at the sensing electrode 10 satisfies a
predetermined inspection standard as compared to a case in which
XPS or AES analysis is performed to directly obtain the Au
maldistribution degree. According to the present preferred
embodiment, one-hundred percent inspection can be performed on the
Au maldistribution degree at the sensing electrode 10 in the
manufacturing process (mass production process) of the sensor
elements 101A.
<Modifications>
The temperature range of 640.degree. C. to 850.degree. C. having
upper and lower limit values at the temperatures of 640.degree. C.
and 850.degree. C., which are used as the sensor drive temperature
in the above-described preferred embodiment, covers the sensor
drive temperatures of various kinds of sensor elements using solid
electrolyte bodies, which are included in gas sensors of any other
kinds such as a sensor element provided in a limiting-current gas
sensor using an electrochemical pump cell, in addition to the
sensor element of a mixed-potential gas sensor as described above.
For example, the sensor drive temperature of a limiting-current NOx
sensor is set to be 800.degree. C. to 850.degree. C. approximately.
Thus, each inspection method according to the above-described
preferred embodiment is applicable, not only to a sensor element
provided in a mixed-potential gas sensor, but also to a sensor
element containing solid electrolyte body as a main constituent
material and including an electrode made of a Pt--Au alloy in which
Au is concentrated on the surface of the noble metal particle.
While the invention has been shown and described in detail, the
foregoing description is in all aspects illustrative and not
restrictive. It is therefore understood that numerous modifications
and variations can be devised without departing from the scope of
the invention.
* * * * *